The long-term goal of the proposed research is to better understand the molecular basis of circadian rhythms in eukaryotes. These studies will be carried out in Arabidopsis thaliana, a model organism well suited to genetic and biochemical approaches. Although putative clock components have been identified in Arabidopsis, the mechanism by which they regulate circadian rhythms is unclear. In addition, the means by which the central clock controls outputs so that they occur at the most advantageous time of day is unknown. The proposed studies use genetic and biochemical approaches to address these fundamental questions. First, a gene responsible for altered circadian regulation will be positionally cloned and other cloning projects will be initiated. Second, the mechanism by which the clock controls phase-specific gene regulation will be explored and a novel clock-regulated transcriptional activator identified. These studies will greatly improve our understanding of clock regulatory mechanisms, information that ultimately may be used to improve human health through treatment of circadian disorders.